Galvanization of wire is carried out commercially by passing the wire through a bath of molten metal, such as zinc. The freshly galvanized wire is particularly susceptible to oxidation at the location where it breaks the surface of the molten zinc bath. In addition, there tends to be a build-up of zinc oxide particles on the surface of the bath and these particles tend to adhere to the wire. To prevent adhesion of zinc oxide particles to the metallized wire it is common practice to employ a layer of charcoal impregnated with oil at the location where the wire breaks the surface of the molten zinc bath. This practice helps to clean the wire of any particles of zinc oxide "ash" loosely adhering to it and also helps to protect the freshly galvanized surface of the wire from oxidation.
There are however a number of drawbacks to this process. The charcoal needs regular replenishment. Erosion of the charcoal reduces the protection against oxidation and the pick-up of zinc oxide particles.
An alternative method of protecting the emerging freshly galvanized wire from oxidation is to employ a shroud around the location where the wire leaves the surface of the molten zinc. A gas that does not react with the zinc to form zinc oxide, such as nitrogen or argon, is passed into the region defined by the shroud, so as to maintain the region around the emerging galvanized wire relatively free of oxygen as compared to air. Applicant has observed that such regions occupied by nitrogen or argon as the protective gas produce better quality wires for a short period of time. It has been found however that over prolonged periods of operation there remains a build up of zinc oxide around the surface of the emerging galvanized wire which has a deleterious effect on the quality of the wire. As a consequence, the weight of the metallized coating may vary and the resulting product may have a rough surface finish.
The build-up of zinc oxide may arise partly as a result of the reaction between the molten zinc and the oxygen present in the atmosphere within the shroud. Zinc oxide may also develop from the reaction between zinc and any fluxing agent which is used to pre-treat the wire so as to facilitate the formation of a good bond between the zinc coating and the ferrous metal. Accordingly, mere maintenance of a relatively non-oxidizing atmosphere by the use of an inert gas alone in the vicinity of the location where the wire leaves the surface of the molten zinc is inadequate to obtain the highest quality finish to the galvanized wire.
There is therefore a requirement for a method and apparatus for treating freshly metallized substrates which avoids the build-up of metal oxide around the surface of the molten metal.